In recent years, with the computerization of automobile systems, the number of engine control units installed in an automobile has been increasing. Meanwhile, with an increase in the needs for energy saving and a reduction in the weight of automobiles, a reduction in the size of automobiles has been desired, and the expectations for flexible printed wiring boards have been increasing accordingly. A flexible printed wiring board is formed by, for example, attaching a copper foil on a front surface side of a base film formed of a polyimide resin, etching the copper foil to form a conductive pattern, and then stacking a cover film by using an adhesive so as to cover the surface of the conductive pattern. This cover film is formed of a polyimide resin or the like and has a function of oxidation prevention, insulation, and protection of the conductive pattern.
However, in the case where a flexible printed wiring board is assumed to be used in an engine control unit in the periphery of an automobile engine, heat resistance equal to or higher than that in other places is necessary. For example, heat resistance at 150° C. is necessary in the periphery of an engine. In existing general-purpose flexible printed wiring boards, polyimides and copper circuits, which are materials of the printed wiring boards, have sufficient heat resistance. However, adhesives used for stacking these components have poor heat resistance, and thus applications of flexible printed wiring boards to the periphery of an engine have not been realized in the present situation. In view of this, adhesives have been examined in order to improve the heat resistance of flexible printed wiring boards (Japanese Unexamined Patent Application Publication Nos. 2011-190425, 2011-157440, 2003-213241, and 2000-226566).